French patent application No. 89 00 009, filed Jan. 2, 1989, corresponding to U.S. Pat. No. 4,983,789 describes a circuit breaker of the above-mentioned type as shown in axial half-section in FIG. 1, and comprising, for each phase: a cylindrical insulating casing (1) filled with SF.sub.6 gas under pressure; a stationary arcing contact (3, 3A); and a moving assembly connected to a drive member and comprising: a moving main contact (5A); a moving arcing contact (4A); a blast cylinder (5) associated with a blast nozzle (6) and co-operating with a first piston (13); a second piston (14) secured to the moving assembly and sliding in a stationary second cylinder (8B). The section of the second piston is large relative to the section of the blast cylinder, said second piston being associated with means (15) for very low head loss communication with the arcing zone, said second piston being pierced by calibrated orifices (14C) to limit the pressure on the face of said piston that receives the gases heated by the arc.
In that circuit breaker, the moving arcing contact (4A) is constituted by a first end of a metal tube (4) coaxial with the casing (1) of the circuit breaker, the second end (4B) of said tube being, connected to the drive member. The second piston (14) is an annular piston outside the tube (4) and secured thereto. The low head loss communication means is constituted by large openings (15) pierced in the periphery of tube (4), the inside of the tube (4) being closed substantially level with the second piston 14 by means of a web (17).
The first piston (13) is semi-moving relative to the second cylinder (8B) and it abuts a slide (11) at one end. The slide (11) can take up two extreme positions in which a second end obstructs or leaves open, openings (10) in the second cylinder (8B) opening out into the volume V0 adjacent to the casing (1).
Those openings are radial chimneys 10.
It may be observed that the piston (14) carries a non-return valve (16) that is urged to close when the pressure on the face of the second piston (14) situated facing the arcing zone is greater than the pressure on the other face of piston (14).